How do condensing boilers work?

Condensing boilers, unlike conventional boilers, are able to utilise the heat contained in the steam produced by combustion.
While in conventional boilers this is lost with the flue gases, in condensing boilers it is captured, allowing the combustion flue gases to cool until the steam condenses as water. This releases the latent heat of condensation, which is recovered by the heat exchanger in the boiler to increase its energy efficiency.